Significant force would have been needed to cause the head injury that killed journalist Phillip Cottrell, a neurosurgeon says.

Mr Cottrell, 43, was known to have a "brittle bones" condition, but his skull appeared to be like any normal, healthy adult's, neurosurgeon Martin Hunn told a High Court murder trial in Wellington yesterday.

He was asked what could have caused Mr Cottrell's head injury, which was a curved break of the skin over a shattered area of skull, with radiating fractures.

Mr Hunn said he had seen similar injuries caused in one case by a vase, and in the other by a hammer, being used with great force.

Nicho Allan Waipuka, 20, and Manuel Renera Robinson, 18, deny murdering Mr Cottrell, a Radio New Zealand bulletin editor, in central Wellington early on December 10 last year. Waipuka has admitted punching him once and taking his wallet.

Mr Hunn agreed a skull fracture such as Mr Cottrell's could occur if someone fell on to a hard surface without breaking their fall, such as after being knocked out with a punch.

But those people almost always fell backwards, he said. It would be unusual for them to fall sideways, as Mr Cottrell did.

Mr Cottrell was found with his head on uneven concrete. That surface could have caused the fracture, but it would have needed something like a stone with a well-defined slightly curved edge to have broken the skin.

Robinson's lawyer says Robinson was on the other side of the road and not involved in the assault.

However, the court has heard that two relatives of Robinson told police after Mr Cottrell died that Robinson said he had kicked and stomped on a man.

In court on Thursday, the relatives changed their minds about what they told police. One said he lied to police and the other said he was not sure.

Mr Hunn was shown two pairs of shoes in court. He said the heels of both, and the toes of one pair, could have caused Mr Cottrell's injuries if used with enough force.

However, one pair was more consistent with the injury than the other.

He said the bone condition osteogenesis imperfecta could have affected the extent to which Mr Cottrell's skull fractured, and may have made bleeding in his brain more likely.

But a piece of skull removed during emergency surgery after the attack did not shatter when it was cut, offered the same resistance to a saw and was of normal thickness to a healthy adult's.

The trial resumes on Monday, exactly a year after Mr Cottrell was injured.